Biology Reference
In-Depth Information
immunoresponse against one virus can be used as carrier proteins for
the neutralizing epitopes derived from the other virus. For example,
HBsAg can be used for the insertion of antigenic epitopes from HIV,
thus providing a multivalent construct that elicits antibodies against
both viruses.
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As noted above, the real protein engineering begins when simple
building blocks with clearly known structure-function connections
are available for the construction of structures with desired properties.
The carrier protein strategy uses building blocks that are relatively
simple but not completely understood. In many cases, the approach
requires a trial of several constructs or expression systems because of
limited understanding of the factors deemed important for vaccine
engineering, such as aggregation, structure stability, and stimulation
of an immunoresponse against the insert.
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Functional strategy
. Application of the structural approach to
protein design is constrained by a limited understanding of the QSAR
for immunological properties of viral proteins. To address this limita-
tion, researchers devised the “functional” strategy for obtaining pro-
teins with desired functions in the absence of structural knowledge.
The basis of this strategy is simple and almost as old as humankind,
i.e. nature's method of evolution. Throughout history, humans have
used a similar approach to breed domestic animals and cultivate
plants. Applied to proteins, the approach became known as directed
evolution (DE), or molecular breeding. Although the DE approach
does not require any degree of rational design, it is helpful in obtain-
ing new proteins and proteins with improved properties, and it is cur-
rently the protein engineering method of choice in many laboratories.
The DE approach involves the development of a library of protein
sequence variants and a procedure for selecting variants with desired
immunological properties (e.g. ability to bind with antibodies). If a
library includes an exhaustive array of sequence variants, then the DE
approach theoretically allows for the selection of all protein structures
that, for example, can bind a particular monoclonal antibody. Such an
exhaustive library can be obtained using random protein sequences.
In the example of monoclonal antibody binding, the power of this
strategy is in the opportunity to identify not only naturally occurring
